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Tornadoes in northeast Iowa, southeast Minnesota, and
southwest and central Wisconsin
Main Tornado page
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| Background Information | Statistics (1850-Present) | Records (1850-Present) |
A scale used to classify the strength of a
tornado. It was devised by Dr. Theodore Fujita from the University of Chicago.
The F-scale gives tornadoes a numerical rating from F0 to F5. The following
table shows the F scale in more detail:
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Wind Speeds |
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Gale Tornado |
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Some damage to chimneys; breaks branches off trees; push over shallow-rooted trees; damage sign boards. |
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Moderate Tornado |
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The lower limit (73 mph) is beginning of hurricane wind speed; peels shingles off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the roads. |
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Significant Tornado |
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Roofs torn off frame houses; mobile homes demolished; boxcars pushed over; large trees snapped or uprooted; light-object missiles generated. |
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Severe Tornado |
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Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown. |
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Devastating Tornado |
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Well-constructed houses leveled; structure with weak foundation blown off some distance; cars thrown and large missiles generated. |
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Strong frame houses lifted off foundations and carried considerable distance to disintegrate; automobiles-sized missiles fly through the air in the excess of 100 m; trees debarked; incredible phenomena will occur |
Since the F-scale is based on tornado damage
(primarily to buildings), there is some ambiguity in the scale. For example,
a tornado which moves over open country will tend to receive a lower rating
than a tornado which strikes a populated area. Since buildings have a wide
variation in age, quality of design, and quality of building materials,
more uncertainties are thrown into the mix. Tornadoes over open country
will probably encounter varying type of vegetation, leading to uncertainties
in these cases.
